How long will it take to transfer my domain to Porkbun?

Max. time for a domain to transfer from one registrar to another:
five days.

ICANN policy allows for transfers that take less than five days, but it’s entirely up to the old (aka “losing”) registrar as to whether the transfer will occur before five days have elapsed. Some registrars always wait the full five days in an attempt to cut down on domain hijack attempts, for instance when the domain owner’s email account is breached. If you're looking to get a domain transferred to Porkbun as quickly as possible, your best bet is to contact your old registrar's support department and request an expedited transfer. It can't hurt to ask!

The five day maximum is set by ICANN’s rules:

“Failure by the Registrar of Record to respond within five (5) calendar days to a notification from the Registry regarding a transfer request will result in a default “approval” of the transfer.”

source:  https://archive.icann.org/en/transfers/policy-12jul04.htm

When you purchase a transfer, there are two ways to initiate the transfer, depending on the domain. The first, is to submit the authorization code provided by your previous registrar. If the domain requires email verification, the transfer will begin when the "Confirmation" link is selected in the Porkbun Transfer Confirmation email. The email will be sent to the Registrant contact listed for the domain in WHOIS. Once the domain is initiated via authorization code or email, it can take up to 5 business days from that point.